Patent GB-924050-A (GB)
Title: Novel active dry yeast products and process for producing the same
Inventor: JOHNSTON WILLIAM REDMOND
IPC: C12N1/04
Filed: 1959-07-27
Abstract: An active dry yeast composition, the particles of which rapidly disintegrate and disperse in water into individual cells, most of which are viable, is characterized by containing less than about 8% moisture by weight and in being in the form of unground fine particles of substantially ovular and spherical shapes, most of the particles are less than 200 microns in diameter and the surface of the particles when viewed under magnification exhibit a uniform assembly of individual yeast cells.  The active dry yeast is prepared by suspending yeast, particularly a bakers' yeast, including yeast which has been conditioned to restrict the protein and phosphorus contents, in a finely divided state in a medium not harmful to yeast which is liquid at the drying temperature both in the presence and absence of water, e.g. an edible vegetable oil, and then drying the yeast by evaporating water from the suspension while maintaining the yeast in suspension in the liquid.  The evaporation may be effected by passing a stream of gas not harmful to yeast, e.g. air, oxygen, nitrogen or carbon dioxide through the yeast suspension or by spraying the suspension repeatedly through an atmosphere of the gas. Specified liquids are edible oils of vegetable, animal or mineral origin, such as corn, soybean, cottonseed, rice, peanut and sperm whale oils which may contain a wetting agent such as sodium mono-oleate, monolaurate or mono-stearate, lecithin, phosphorylated lecithin or the diacetyl tartaric acid ester of 95% lard monoglyceride; silicone liquids such as a liquid water-immiscible siloxane polymer containing methyl and ethyl groups; and certain liquid esters such as dibutyl phthalate, tributyl citrate and triethyl citrate.  It is stated that some liquids, such as acetone and hydroxylic liquids, such as ethanol, glycerol, propylene glycol and 2,4-pentadiol have been found unsuitable as suspending liquids since they injure the viability of the yeast cells.  Initial drying temperatures of between 95 DEG  and 104 DEG  F. are preferred until the moisture content of the yeast product is reduced to about 20%; as drying continues, the temperature may be increased gradually e.g., temperatures up to about 120 DEG  F. when the yeast product contains about 10% moisture, up to about 140 DEG  F. when the moisture content is about 5% and up to about 155 DEG  F. when the moisture content reaches about 2%.  Upon completion of the drying, the yeast may be separated from the bulk of the oil (or other suspension liquid) by settling, filtration or centrifuging, to give a dry yeast composition with a residual oil content of 10% to 5%.  The level of residual oil may be reduced to below 1% by extraction with a solvent such as methylene chloride, hexane, ethanol and acetone to yield free-flowing particles.  A saccharide capable of being fermented by yeast such as a mixture of yeast fermentable and non-fermentable sugars or a hydrolysed starch syrup (D.E. 42 to 65) containing dextrose, maltose, dextrins and higher sugars may be added to the suspending liquid, to the yeast or to the mixture of the two preferably before drying; the active dry yeast composition produced will exhibit substantially the same leavening activity when incorporated in dough without previous rehydration as when incorporated in dough after previous rehydration, by having when fresh, substantially the same leavening activity, based on yeast solids content, as fresh wet compressed bakers' yeast of the Bios No. 236 strain and by retaining such high leavening activity under like storage conditions many times longer than the compressed yeast.  In the case of hydrolysed starch syrup, the proportion added is preferably adjusted so that the dry yeast composition obtained includes a residue, amounting to at least 25% of the composition, of dextrin and sugar constituents not fermentable by yeast and being substantially free from yeast fermentable constituents except maltose.  When drying the yeast in the presence of a fermentable sugar, moisture in excess of 60 to 65% by weight of the oil-free product should be evaporated in a period of about one hour, and further reduction of the moisture content to about 20% completed is not less than an additional 1 1/2 hours; the rate at which the remainder of the moisture is removed does not appear to have a great influence upon the leavening activity of the final product.  According to one of many examples (Example 3), yeast of the Gebruder-Mayer strain (Bios No. 23) was propagated to condition it for drying in accordance with standard practice by restricting its nitrogen and phosphorus nutriment during propagation and then separated from the wort by centrifuging. 350 millilitres of cream yeast thus obtained, containing 80.7 grams of yeast solids, were dispersed in 5 litres of corn oil. Air at a temperature of 77 DEG F., having a relative humidity of 20%, was passed through the dispersion at the rate of 300 cubic feet per hour for 6 hours, the yeast-oil dispersion being maintained at approximately 86 DEG  F. The yeast was then separated from the bulk of the oil.  The dry yeast product thus obtained contained about 18% oil and had a moisture content of 5.6% calculated on an oil-free basis; it exhibited good leavening activity and storage stability.